This invention relates to a pellicle structure used for the protection of photo masks and reticles in microlithography applications and, more specifically, relates to a novel pellicle structure which can transmit an average of about 90% or greater over a radiation band width beginning at about 240 nanometers.
Pellicles, which are free standing thin transparent organic films are commonly used to protect photo masks and reticles in order to increase die yield in microlithography during the manufacture of semiconductor devices. Such pellicles are normally attached to a mask used in the microlithography application and stand above the mask suface by a given distance. Thus, dust particles which may settle during the microlithography operation will settle on the pellicle and be defocused on the work surface. Consequently, when used in a conventional photolithographic apparatus, one or more dust particles will not affect the yield of a given wafer. Die yield from such wafers is improved by up to and greater than 40% when the mask and/or reticle are pellicle protected.
Pellicles of this type are described in a publication by R. Hershel, Semiconductor International, 8, 97, (1981). Their use is also described in a publication by A. Rangappan, C. Kao, "SPIE Optical Microlithography", Vol. 334, 52, (1982). Also, see I. Ward, Dawn Duly, SPIE, Vol. 470, Optical Microlithography III: Technology for the Next Decade (1984), pp. 147-156.
A standard pellicle being sold today, for example by the assignee of the present application, consists of a membrane which is commonly of nitrocellulose having a thickness of about 2.85 microns. Such nitrocellulose pellicles satisfy the light transmission requirements for most present microlithography applications. Films other than nitrocellulose are also known for use as pellicles. By way of example, films have been made of materials such as mylar, cellulose acetate, and parylene (poly(chloro-p-xycylene)). Other materials are also known and the foregoing is not an exhaustive list. Nitrocellular pellicles do not transmit sufficient light at shorter wavelengths, for example below 360 nanometers. However, there is a growing need for pellicle materials which can transmit useful amounts of an input light in the mid range of ultraviolet wavelengths, particularly in the 280 to 360 nanometer range. Such materials must also permit relatively economical manufacture and have mechanical durability which will enable them to exist as a free standing membrances which are not too rapidly degraded by ultraviolet radiation and can withstand at least minimal mechanical abuse during handling and cleaning. Moreover, the pellicle film must be capable of being made thin enough and of sufficient uniformity so as to not interfere with the optical properties of the microlithographic process.